Microfluidic devices find application in micro total analysis systems (μTAS) or lab-on-a-chip (LOC) systems because such devices offer the ability to analyze small sample volumes, and can be developed into highly parallel systems at reduced costs. In particular, such systems can be used in biological and clinical applications in which particle manipulation is used to perform operations, for example, concentrating, detecting, sorting, and focusing particulate samples, such as cells and colloids. Passive manipulation of particles flowing through microfluidic devices, by techniques such as hydrodynamic focusing, size filtration, and sedimentation, is relatively simple in comparison to active manipulation using external energy such as optical forces, magnetism, electro-kinetics, dielectrophoresis, acoustics, and the like. Passive manipulation does not rely on external sources of energy, but instead can be accomplished using geometries of micro-channels in devices, and flow conditions through such channels. In contrast, active manipulation can employ external sources of energy and can require the integration of powered components to the microfluidic devices.